I do not know of any websites to do what you are requesting.
But it would seem to me that you need two things.
1. Books with Certification information, sample exams, etc.
2. A place to actually do the coding so you have practical experience, For that you would need to find al place that does AS400 timeshare. That way you machine would have the latest release for you to do testing. One example is http://timeshare400.com/